The most simple way to explain her would be The Norse Goddess of Death, who rules the realm of Hel, which is where people who weren't great fighters go after they die. (It's not Hell, it's Hel. Not all people suffer there for eternity, Hela judges them and give them what they deserve).
As a goddess of Death she's sometimes a villain, sometimes she helps the heroes. She is mostly on her own side and will help whoever she sees will benefit her the best.
She's also very lonely.
As for her relationship with Loki- it's complicated. It was hinted several times that he's her father, but it was also shown that she was an adult when he was still a child. In a recent story it was basically explained that he 'created' her in the future, but sent her to the distant past to protect her. So he's 'sort of'' her father, but not really.